This matches current Crankshaft/fullcodegen behavior more closely and
thus reduces the chances that we run into unnecessary polymorphism due
to the field representation tracking in our object model.

R=jarin@chromium.org
BUG=v8:5267

Committed: https://chromium.googlesource.com/v8/v8/+/6a939714e991ebf10d56ddbd2869325cca99c0ef
Review-Url: https://codereview.chromium.org/2367593003
Cr-Commit-Position: refs/heads/master@{#39692}